This invention relates to optical recording and reproducing devices for recording information on and reproducing it from an optical disk, and more particularly to optical heads thereof provided with a servo mechanism for correcting the tilting of the optical axis of the light beam with respect to the recording surface of the optical disk.
Optical disk recording and reproducing devices are now widely used for information storage. In such devices, information is stored on spiral tracks formed on an optical disk and is recorded and read by a laser light beam focused on the disk by an optical head. In order to ensure that the light beam is converged correctly on the track, the optical heads must be provided with servo mechanisms for correcting focusing and tracking errors thereof. Further, they must be provided with a tilt servo mechanism; namely, if the optical axis of the light beam radiated from the optical head is not at right angles to the information recording surface of the optical disk, an aberration called a coma may result, and cause crosstalk between the adjacent tracks. In order to prevent comas, the optical heads must be provided with a servo mechanism for correcting the tilting of the light beam away from the direction at right angles to the recording surface of the optical disk.
FIGS. 1 through 7, show the structure of an optical head provided with a conventional servo mechanism for correcting the tilting of the light beam as well as for correcting focusing and tracking errors.
FIG. 1 shows the overall organization of an optical head device of an optical disk recording and reproducing device. The optical head device utilizes the twin spot method and the astigmatic method (described below) for detecting tracking and the focusing errors, respectively. A laser diode 1 constitutes a light source for emitting a divergent pencil of rays 2. The emitted pencil of rays 2 is separated by a diffraction grating 3 into three pencils of rays 2a, which are reflected by a beam splitter 4, collimated into parallel pencils of rays 2b by a collimating lens 7, and converged by a converging lens 5 at spots 9a, 9e and 9f, respectively, along a track 8 on an optical disk 6. The disk 6 is rotated by a motor 60 and comprises a transparent substrate 6a and a recording surface 6b on which information recording tracks 8, 8a and 8b are formed. The rays of light reflected at the spots 9a, 9e, and 9f along the track 8 are directed via the converging lens 5 and collimating lens 7 to the beam splitter 4 for reparation from the emitting pencils of rays 2b and fall upon a photodetector 10 as three reflected pencils of rays 2c. The beam splitter 4 comprises, in addition to a partially silvered surface for separating the reflected rays 2c from the emitted pencil of rays 2a, a grating having a grating interval that varies gradually within the opening thereof to give an astigmatic abberration (astigmatism) to the reflected rays 2c that are diffracted light of first order.
FIG. 2 shows a vertical section of the converging lens 5. The converging lens 5 comprises a lens portion 5a for converging the emitted and reflected pencils of rays 2b and 2c, a flange 5b for supporting the converging lens portion 5a, and an attachment 5c for connecting the lens portion 5a to actuators 5d and 5e (described below) via the flange 5b. These portions 5a through 5c of the converging lens 5 are formed as a single integral piece.
FIG. 3 shows a slightly modified optical head device wherein the collimator lens 7 of the device of FIG. 1 is not utilized; otherwise, the optical disk device of FIG. 1 is similar to that of FIG. 1 both in its structure and method of operation, wherein like reference numerals represent like or corresponding parts.
As shown in FIG. 4, the photodetector 10 comprises a central portion partitioned into four cells A through D, and a pair of lateral cells E and F. The central pencil in the emitted rays 2b which falls on and is reflected at the information reading spot 9a on the track 8 posses through the converging lens 5, the collimating lens 7, and the beam splitter 4, and then falls on the central portion consisting of cells A through D as the central information carrying pencil of rays 20a (shown by a cross-hatched circle in the figure). The lateral pencils in the emitted rays 2b falling on and reflected at the spots 9e and 9f pass through the converging lens 5, the collimating lens 7, and the beam splitter 4, and then fall on the lateral cells E and F as the pencils of rays 20e and 20f for determining the tracking error. The tracking and focusing errors are adjusted by the tracking actuator 5e and the focusing actuator 5d, respectively, in response to the signals generated by the circuit shown in FIG. 4, on the principles of the twin spot method and the astigmatic aberration method, respectively, as described below.
The tracking error may be caused by the fact that the center of the optical disk 6 deviates from the center of rotation of the motor 60 due to mounting error, etc. Such tracking error is detected and adjusted by the twin spot method as follows.
The pencil of rays 2 emitted from the laser diode 1 is separated by the diffraction grating 3 into three pencils of rays (i.e. a central pencil and a pair of lateral pencils of rays) which are converged on spots 9a, 9e and 9f along an information recording track 8 as described above. Since the information is carried by the central pencil of rays 20a reflected from the central light spot 9a, the spot 9a should be positioned precisely on track 8. Thus, the line connecting the converging light spots 9a, 9e and 9f is slightly inclined with respect to the direction of the track 8. The pencils of rays reflected diffusively at spots 9e and 9f are converged via the converging lens 5, the collimating lens 7 (which is omitted in the case of the device of FIG. 3), and the beam splitter 4 on the cells E and F of the photodetector 10 as lateral pencils of rays 20e and 20f. The subtractor 13 calculates the difference between the outputs of the two lateral photodetector cells E and F, and the phase compensation circuit 14 outputs in response to the differential output of the subtractor 13 a tracking error signal V.sub.AT corresponding to the tracking error, i.e. the deviation of the information reading spot 9a from the track 8 in the X direction perpendicular to the Y direction of the track 8. In response to the tracking error signal V.sub.AT, the tracking actuator 5e adjusts the position of the converging lens 5 in the X direction perpendicular to the track Y direction (as indicated by arrows AT in FIG. 3), so as to position the information reading spot 9a correctly on the center of the track 8.
On the other hand, focusing error may be caused by the fact that the surface of the optical disk 6 is uneven, which may result in an undulating motion of the surface when the disk is rotated. Focusing error is detected and adjusted by the astigmatic method as follows.
As described above, the beam splitter 4 gives an astigmatism to the central pencil of rays 20a reflected from the information reading spot 9a. The photodetector 10 is positioned in the Z direction perpendicular to the surface of the optical disk 6 at a location at which the central pencil of rays 20a falls on the central portion (i.e. the cells A through D) of the photodetector 10 as a substantially circular radiation spot of minimum blur, as shown by cross-batching in FIG. 4, when the light spot 9a is focused precisely on the track 8. As the spot 9a on the optical disk 6 becomes too near to or too far away from the optical head (i.e., the converging lens 5) to cause a focusing error, the pencil of rays 20a on the central photodetector cells A through D is deformed into an oblong elliptic form, elongated either in the diagonal direction across cells A and C or in the other diagonal direction perpendicular thereto across cells B and D, as shown by dotted curves in FIG. 4. A subtractor 12 having an input terminal coupled to the outputs of cells A and C and another input terminal coupled to the outputs of cells B and D calculates the difference (A+C)-(B+D) between the sum (A+C) of the outputs of cells A and C and the sum (B+D) of the outputs of cells B and D. In response to the output of the subtractor 12, the phase compensation circuit 15 outputs a focusing error signal V.sub.AF corresponding to the focusing error of the spot 9a in the Z direction of the optical axis O perpendicular to the surface 6b of the optical disk 6. In response to the focusing error signal V.sub.AF, the focusing actuator 5d adjusts the position of converging lens 5 in the Z direction of the optical axis O so as to correct the focusing error. As is well known, it is preferred that the direction of astigmatism generated by the plate-shaped beam splitter 4 form an angle of 45 degrees with the Y direction of the tracks 8, 8a, and 8b. Thus, the laser diode 1 is positioned such that the pencil of rays 2 emitted therefrom forms an angle of 45 degrees with the Y direction of the tracks on the optical disk 6.
The sum (A+B+C+D) of the outputs of the four central photodetector cells A through D, which is calculated by an adder 11, is utilized as the information reproducing signal V.sub.RF in an information reproducing circuit (not shown) of the optical disk recording and reproducing device.
As described above, the focusing and the tracking error are detected by the astigmatic method and the twin spot method, respectively, whereby a single light source, i.e. the laser diode 1, is utilized both in reading out information from the optical disk 6 and in detecting the focusing and tracking errors. Thus, the structure of the device for the detection of such errors is relatively simple. However, as described below in reference to FIGS. 5 and 6, the conventional optical head device is in need of a separate light source for detecting the tilting of the optical head with respect to the optical disk.
The tilting of the optical axis O (see FIGS. 1 and 3) of the optical head, i.e. the converging lens 5, from the Z direction perpendicular to the recording surface 6b of the optical disk 6 may be caused either by the bending of the transparent substrate 6a of the optical disk 6 or by the slanting of the rotational axis of the optical disk 6 from the Z direction. When this tilting of the optical axis of the optical head with respect to the recording surface 6b of the optical disk 6 occurs, an aberration called a coma results, which may cause crosstalk from the tracks 8a and 8b adjacent to the current information reading or recording track 8 on which the information is to be recorded and reproduced by the optical head. Such crosstalk present an especially serious problem in the case of devices such as optical video disks where the information is recorded as an analog signal.
FIG. 5 shows the structure of a conventional tilt servo mechanism for correcting the tilting of the optical head with respect to the surface of the optical disk 6. A light emitting diode 30 and a pair of photodetectors 31 and 32 are mounted on the head supporting box 33 which accommodates the above described optical head device therein. The box 33 is rotatably supported by a support member 33a via a rotational shaft 33b. The light emitted from the light emitting diode 30 is reflected by the recording surface 6b of the disk 6 and falls on the photodetectors 31 and 32. A subtractor 34 calculates the difference between the outputs of the two photodetectors 31 and 32. In response to the differential output of the subtractor 34, a phase compensation circuit 35 outputs to a motor 36 a signal corresponding to the amount of the tilting of the optical head with respect to the recording surface 6b of the disk 6. Thus, in response to the tilt signal from the circuit 35, the motor 36 drives the head supporting box 33 around the shaft 33b via a screw 37 to adjust the angle of the box 33 with respect to the surface of the disk 6.
The output of the two photodetectors 31 and 32 are equal when the optical axis of the optical head accommodated in the box 33 is perpendicular to the recording surface 6b of the disk 6. When the recording surface 6b of the disk 6 is tilted around an axis running in the direction of the tracks on the surface 6b, the amount of light incident on the two photodetectors 31 and 32 becomes uneven, as shown in FIG. 6. Thus, the output of the subtractor 38 becomes positive or negative according to whether the recording surface 6b of the disk is tilted in one direction or the other. Consequently, the output of the phase compensation circuit 35 corresponds to the amount of the tilting, and the box 33 is adjusted via the motor 36 in such a manner that the optical axis of the optical head accommodated in the box 33 is maintained at right angles with the recording surface 6b of the disk 6.
In the case of the devices shown in FIGS. 1 and 3, the photodetector 10 is positioned at a point on the optical axis O of the converging lens 5. Thus, as shown schematically in FIG. 7(b), the light emitted from the laser diode 1 and reflected by the beam splitter 4 is converged on the recording surface 6b on the disk 6, while the light reflected from the recording surface 6b of the disk 6 is converged on the photodetector 10 after passing through the beam splitter 4. However, the positions of the laser diode 1 and the photodetector 10 with respect to the beam splitter 4 may be reversed, as shown in FIG. 7(a), so that the light emitted from the laser diode 1 which lies on the optical axis O of the converging lens 5 passes through the beam splitter 4 and converged on the recording surface 6b on the disk 6. In this case, the light reflected from the recording surface 6b of the disk 6 and converged by the converging lens 5 is reflected by the beam splitter 4 before falling on the photodetector 10.
As described above, conventional optical head devices are provided with a separate light source for the tilt servo mechanism, and hence have the problem that the need for the provision of a separate light source for the tilt detection results in additional optical parts and an additional cost furthermore, since the space between the upper surface of the head supporting box 33 and the optical disk 6 is small, the disposition of the light emitting diode 30 and the photodetectors 31 and 32 thereon imposes a limitation on the design of the optical head device.